I switched my version from 1.8.1 to 1.5.1 to use a resource pack that a youtube builder used. (recourse pack that i switched to: http://bit.ly/1dC3Dp2) And after I had finished downloading and doing everything so that I could use it. I clicked my main private world and over 60 or 70% has been destroyed or maimed in some way. Over 2 years of building random stuff got destroyed. So I naturally switched back to what I had been using thinking that it was just a bug with the resource pack not registering blocks. But this did not fix the issue I still have over half my stuff either partially destroyed or missing completely.
Mojang really should have done something to prevent older versions from loading worlds created in newer versions, and by that i mean not even being able to recognize them, not just warn you (as they added in 1.9, which also only work for that version and newer). This would have been easy to add, actually; the game saves an NBT version, currently 19133, in level.dat and if I change it with NBTExplorer the save no longer shows up (the previous version was 19132, which will display a "must be converted" message since that was for the pre-Anvil format, Conversely, I once loaded Beta 1.7.3 and it did not see any of the worlds I had created in newer versions).
There is also user error here - did you really think that 1.5.1 would be able to load a world with blocks, items, and mobs that did not exist back then, or the alterations they have done to data formats (all items in a 1.8+ world, even those that existed before 1.8, are not recognizable by older versions because they changed the way item IDs are saved)? Mojang has always said that they will never support downgrading; the warning message that comes up in the launcher when you enable Alpha/Beta versions (something like "use at your own risk, may corrupt your worlds") should also appear for any versions older than the latest release, which should be the only version enabled unless you check a box to enable older releases (the message should also appear anytime you select an older version, not just when you check the box).
That said, there is no way you can recover the world unless you have a backup since the game autosaves chunks almost as soon as they are modified (once every 30 seconds) and every time you pause the game (this includes some in-game actions like editing a sign or written book) - which you should always have anyway; you might as well have lost it due to a power surge, hard drive crash, or any of the other numerous ways data loss occurs. However, if you have it enabled you may be able to go into the saves folder (on Windows, %appdata%, then .minecraft\saves) and right-click on the folder for your world and see if you can restore it to a previous version; a more extreme way might be to do a system restore to a previous restore point but I do not think that affects the appdata folder, only system files.
Depending on just how much was damaged you may be able to use Creative (open to LAN and turn cheats on if this is a Survival world without them) or MCEdit, etc to help rebuild and restore any valuable items that were lost; any chunks that were replaced can not be restored though so you'll have to completely rebuild anything that was in them (if the world was created in 1.8 you could delete them with MCEdit so they regenerate with 1.8 terrain so they at least match; I do not recommend regenerating them in any other version even if the damage has already been done).
On a positive note, this will have only affected chunks that were loaded in 1.5.1, which will be limited to a 25x25 chunk area around the world spawn point and a 21x21 chunk area around wherever you were and/or traveled (regardless of render distance, which only controls how many chunks are rendered in versions before 1.7.4, unless you used Optifine, which can load more chunks).
I switched my version from 1.8.1 to 1.5.1 to use a resource pack that a youtube builder used. (recourse pack that i switched to: http://bit.ly/1dC3Dp2) And after I had finished downloading and doing everything so that I could use it. I clicked my main private world and over 60 or 70% has been destroyed or maimed in some way. Over 2 years of building random stuff got destroyed. So I naturally switched back to what I had been using thinking that it was just a bug with the resource pack not registering blocks. But this did not fix the issue I still have over half my stuff either partially destroyed or missing completely.
Mojang really should have done something to prevent older versions from loading worlds created in newer versions, and by that i mean not even being able to recognize them, not just warn you (as they added in 1.9, which also only work for that version and newer). This would have been easy to add, actually; the game saves an NBT version, currently 19133, in level.dat and if I change it with NBTExplorer the save no longer shows up (the previous version was 19132, which will display a "must be converted" message since that was for the pre-Anvil format, Conversely, I once loaded Beta 1.7.3 and it did not see any of the worlds I had created in newer versions).
See also: http://www.minecraftforum.net/forums/minecraft-discussion/recent-updates-and-snapshots/2201993-so-what-happens-if-you-create-something-in-1-8-and
There is also user error here - did you really think that 1.5.1 would be able to load a world with blocks, items, and mobs that did not exist back then, or the alterations they have done to data formats (all items in a 1.8+ world, even those that existed before 1.8, are not recognizable by older versions because they changed the way item IDs are saved)? Mojang has always said that they will never support downgrading; the warning message that comes up in the launcher when you enable Alpha/Beta versions (something like "use at your own risk, may corrupt your worlds") should also appear for any versions older than the latest release, which should be the only version enabled unless you check a box to enable older releases (the message should also appear anytime you select an older version, not just when you check the box).
That said, there is no way you can recover the world unless you have a backup since the game autosaves chunks almost as soon as they are modified (once every 30 seconds) and every time you pause the game (this includes some in-game actions like editing a sign or written book) - which you should always have anyway; you might as well have lost it due to a power surge, hard drive crash, or any of the other numerous ways data loss occurs. However, if you have it enabled you may be able to go into the saves folder (on Windows, %appdata%, then .minecraft\saves) and right-click on the folder for your world and see if you can restore it to a previous version; a more extreme way might be to do a system restore to a previous restore point but I do not think that affects the appdata folder, only system files.
Depending on just how much was damaged you may be able to use Creative (open to LAN and turn cheats on if this is a Survival world without them) or MCEdit, etc to help rebuild and restore any valuable items that were lost; any chunks that were replaced can not be restored though so you'll have to completely rebuild anything that was in them (if the world was created in 1.8 you could delete them with MCEdit so they regenerate with 1.8 terrain so they at least match; I do not recommend regenerating them in any other version even if the damage has already been done).
On a positive note, this will have only affected chunks that were loaded in 1.5.1, which will be limited to a 25x25 chunk area around the world spawn point and a 21x21 chunk area around wherever you were and/or traveled (regardless of render distance, which only controls how many chunks are rendered in versions before 1.7.4, unless you used Optifine, which can load more chunks).
TheMasterCaver's First World - possibly the most caved-out world in Minecraft history - includes world download.
TheMasterCaver's World - my own version of Minecraft largely based on my views of how the game should have evolved since 1.6.4.
Why do I still play in 1.6.4?